Fuel injection in an internal combustion engine can be effected either by providing a fuel injection nozzle to each cylinder of the engine or by providing a single fuel injection nozzle in a throttle body which is common to more than one cylinder. Favorable atomization of the fuel is highly desirable in any fuel injection system, and in the case of the throttle body fuel injection it is possible to improve the quality of the atomization of fuel by providing an air valve upstream of the throttle valve and increasing the air speed in the intake bore of the throttle body so that fuel may be injected into the air flow of relatively high speed resulting from the narrowing of the intake bore by the air valve.
For instance, U.S. Pat. No. 4,378,000 teaches such an air valve which can be opened by the increase in the air flow against the biasing force of a spring which tends to close the air valve. Alternatively or additionally, it is possible to use the negative pressure existing in the intake manifold for the purpose of driving the air valve and in this case an action similar to that of a variable venturi structure which can be seen in some of conventional carburetors can be accomplished.
However, since the opening angle of the air valve is substantially determined by the opening angle of the throttle valve, the atomization of the fuel tends to be insufficient when the engine has not been warmed up. It is possible to provide an electric heater in the intake bore of a throttle body so as to promote the evaporation of fuel in the intake bore as proposed by Japanese Patent Laying Open Publication No. 56-855555, but it makes the structure of the throttle body very comples and therefore expensive to manufacture.